Heat transfer characteristics of a non-premixed turbulent flame formed in a curved rectangular duct

Abstract

Heat transfer characteristics of a non-premixed turbulent flame formed in a curved rectangular duct (180° bend) were investigated experimentally. Key turbulence quantities of velocity and thermal fields such as Reynolds stress components and turbulent heat fluxes were measured using a combined LDV and fine-wire thermocouple technique. These measurements provided direct evidence of the occurrence of the anomalous phenomenon of counter-gradient heat transfer, which can be ascribed to the presence of a strong pressure-gradient in the radial direction of the curved duct. The experiment also revealed that the onset region of this “counter-gradient” diffusion was adjacent to the strong “gradient” diffusion region. The quantitative appraisal of the production terms for the turbulent heat flux showed that the pressure gradient promoted gradient diffusion on the inner-wall (low-pressure) side of the curved-duct flame and caused counter-gradient diffusion on the outer-wall (high-pressure) side. The schlieren photography for visualizing the density field showed a totally different behavior of the burned gas parcels between the high- and low-pressure sides of the flame. The essential mechanism causing the counter-gradient diffusion can be explained by the unique motion of the high-temperature (low-density) gas parcel on the high-pressure side of the flame. High-temperature fluid motions tend to be preferentially damped by the pressure gradient imposed on the flow field. The occurrence of the counter-gradient diffusion phenomenon will of course lead to the collapse of the “gradient-diffusion hypothesis,” on which most conventional turbulence models rely. In such a field, the analogy between heat and mass transfer processes, which holds almost always in normal turbulent passive-scalar transport, can disappear.